The electronics industry is constantly challenged with the need to reduce the size and weight of electronic devices while increasing their performance. One method of providing the necessary increase in density is to combine functions of the different components. Printed Wiring Boards (PWB) are currently used to connect different devices in order to perform an electronic function. It would be desirable if the PWB had some of the functionality of some of those devices, eliminating the need to mount them to the surface. Resistors and capacitors are very simple devices that are required in almost every circuit. Because of the simplicity of their functions, they are prime candidates for incorporation into a PWB structure.
The desire to incorporate a capacitor into a PWB is not a new concept. Limitations in materials, however, have prevented this from becoming common. There are presently several high dielectric materials which can be used as an internal capacitor, but they all have limitations in the amount of capacitance that they can supply. The capacitance of the layer is a function of the thickness and dielectric constant. An increase in dielectric constant and/or a reduction in thickness will increase the capacitance of the layer.
Copper clad substrates have been made with an epoxy/glass core as thin as 2 mils. This core material has a dielectric constant of 4.5. The dielectric constant is material dependent, and the material cannot be made thinner than 2 mils and remain practical for manufacturing. So this type of material is limited to a maximum capacitance of around 500 picofarads/in.sup.2.
Copper clad substrates have been made with a resin/ceramic core as thin as 10 mils. This core material can have a dielectric constant as high as 10. The dielectric constant is material dependent, and the material cannot be made thinner than 10 mils and remain practical for manufacturing. So this type of material is limited to a capacitance of around 220 picofarads/in.sup.2.
Copper clad substrates have been made with a teflon/ceramic core as thin as 5 mils. This core material can have a dielectric constant as high as 10. The dielectric constant is material dependent, and the material cannot be made thinner than 5 mils and remain practical for manufacturing. So this type of material is limited to a capacitance of around 450 picofarads/in.sup.2. Additionally, these types of materials have historically displayed very low voltage breakdown, and, as such present poor reliability as a capacitor. The limitation on thickness generally results from the need to use the material as a copper clad substrate. Thin materials are prone to damage during the printed circuit fabrication process. It would be desirable to use the material as an adhesive sheet (referred to as a prepreg or "B" stage) instead of a copper clad substrate (referred to as "C" stage). The metal layers would be provided by other substrates in the fabrication of the PWB. So the dielectric would then be a thin adhesive bond line between two metal plates. This would facilitate fabrication as well as allowing for thinner layers that would have higher capacitance.
The three copper clads listed above all have limitations in making thin adhesive sheets (prepregs).
Epoxy/glass prepregs use a woven glass reinforcement to provide dimensional stability. Currently, there is a limitation in the manufacturing of a woven glass material which ultimately limits the thickness of the prepreg. The glass does not compress during lamination between two metal planes, so the thickness is limited to the thickness of the woven glass. In addition, the dielectric constant is limited by the materials to a high of about 4.5.
High dielectric constant resin/ceramic prepregs are not currently available because of difficulty in handling. High levels of ceramic are needed to raise the dielectric constant, and this level of filler in a semi-cured or "B" staged material is too brittle to be used as a large thin sheet adhesive.
PTFE/ceramic prepregs have limitations in processing as well as properties. The PTFE requires very high temperatures and pressures that are not compatible with standard PWB processing. In addition, the material has poor dielectric breakdown voltage that changes with time.
Thus a need exists for a prepreg film having a high dielectric constant, at least 4.5, that avoids the drawbacks of the prior art and is an unclad, i.e., nonlaminated, uncured material that can be bonded to inner layers of circuitry in a convenient fashion, e.g., a prepreg.
The subject invention, described below satisfies these needs and avoids the disadvantages of the prior art.